It is known that various metals may be superplastically formed, into various shapes, under elevated temperatures while under pressure. It is also known that various metals may be diffusion bonded together under elevated temperatures while under pressure. In U.S. Pat. No. 3,920,175 to Hamilton et al, these two processes were combined to superplastically form and to diffusion bond all in the same process. U.S. Pat. Nos. 3,996,019 and 4,071,183 to Cogan call for a method of using two superplastically formed sheets with reinforcing inserts retained between the sheets by diffusion bonding to generate a part requiring a minimum of post forming machining.
When superplastic forming in combination with diffusion bonding one uses at least one part that is in the form of a sheet that is deformed under superplastic conditions to define a complex shape. This formed sheet is pressed aginst one or more parts, which may be considered as reinforcing parts, to which the sheet is to be joined. Difficulties are often experienced due to the plastic characteristics of the reinforcing part at the conditions required for the process. Even when the reinforcing part is located around the inside surface of a mold in which the process is taking place, thereby allowing the superplastic sheet to press against and force the reinforcing part against the side of the mold, difficulties are experienced with maintaining tolerance due to flow of the reinforcing part. This is corrected to some extent by utilizing a reinforcing part that is much thicker than the sheet. When the reinforcing part is located in the mold such that the reinforcing art is unsupported on two opposite sides the flow of that part is even more of a problem. To help correct this situation it is necessary to use a large cross sectional area. When one uses a thin reinforcing part that is unsupported on two opposite sides a completely unsatisfactory part is generated as the reinforcing part tends to tip sideways. It was found that titanium parts may be superplastically formed and diffusion bonded while maintaining dimensional tolerances.